The rise of smartphone car booking apps has led to a boom in minicabs in the capital, with the number of driver licences issued by Transport for London rising from 61,200 in 2010 to 117,808 in February this year. Uber itself has more than 30,000 drivers in London.

Uber under fire for alleged tax avoidance

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While travellers have benefited from cheaper taxi fares, Mr Khan has faced pressure to cut the number of minicabs, after complaints from London’s traditional black cab drivers, as well as concerns about congestion.

However, TfL is unable to place a cap on the number of licences issued, and critics claim that the tests, which include essays on subjects such as the the aurora borealis and river pollution, are a ploy to reduce the number of minicabs on the road.

Documents submitted to the High Court by TfL show that the English tests, which will include writing, speaking, reading and listening exams, are likely to reduce the number of private hire drivers to 67,101 over the next three years, a fall of 43 per cent.

Transport sources suggested that the impact of the huge drop in minicabs would make it harder for customers to find a car, as well as leaving them with longer waits for drivers to arrive.

But Mr Khan said that forcing drivers to have a better grasp of English “and understand information from passengers and licensing requirements is a vital part of ensuring passengers get the high standard of service they need and deserve”.

The mayor added: "This could include discussing a better route, talking about a medical condition, or ensuring every driver is fully up to date with new regulations.”

Uber argued that the proposals would have a disproportionate impact on drivers from countries where English was not generally spoken and give rise to "indirect discrimination on grounds of race and nationality".

But Mr Justice Mitting said TfL was entitled to require drivers to demonstrate compliance with the English language requirement and there was not any practical alternative.

London taxpayers are expected to face a legal bill of around £200,000 after the judge upheld three other areas of Uber’s claim, quashing requirements for firms to have a 24-hour phone line and for drivers to be required to have commercial insurance even when they were not carrying passengers.

The company said it would appeal the judgement. Tom Elvidge, general manager of Uber in London, said: "While we are glad the court agreed with us on the other measures TfL tried to impose this is a deeply disappointing outcome for tens of thousands of drivers who will lose their livelihoods because they cannot pass an essay writing test.

"We've always supported spoken English skills, but writing an essay has nothing to do with communicating with passengers or getting them safely from A to B.

"That's why we intend to appeal this unfair and disproportionate new rule."